It is known that at present there is an ever increasing tendency to construct the domes of electric arc furnaces in a composite manner, as indicated above, because this new arrangement is more economical. It will easily be understood that it is not possible to construct an electric furnace dome entirely of metal because of the particularly intense variations in field at the level of the passage of the electrodes. The large amount of energy which is dissipated at this level in the form of heat leads to the necessity of making this portion of the cover of a refractory material. On the other hand, the peripheral portion of the dome where the variations in field are much less can be metallic, on condition that it comprises a cooling circuit.
Domes thus constructed are known, but they have serious disadvantages principally because they are made plane, so that every time there are sudden variations in the volume of gas emitted during the production of the steel, fumes laden with dust and gas, such as carbon monoxide, are released outside the furnace.